1. Field of the Invention
The present invention relates to a ladder-type piezoelectric filter.
2. Description of the Background Art
FIG. 13 is a circuit diagram of a conventional ladder-type piezoelectric filter 8. As shown in FIG. 13, the ladder-type piezoelectric filter 8 is made up by alternately connecting parallel arms 81, 83, 85 and series arms 82, 84. The parallel arms 81, 83, 85 are respectively made up of parallel resonators 811, 831, 851, and the series arms 82, 84 are respectively made up of series resonators 821, 841. In the ladder-type piezoelectric filter 8, antiresonant frequencies of the parallel resonators 811, 831, 851 are substantially matched with resonant frequencies of the series resonators 821, 841, so as to obtain a band pass characteristic.
However, a pass band width of the ladder-type piezoelectric filter 8 depends upon the difference between the resonant frequency and antiresonance frequency of the parallel resonators 811, 831, 851 and the series resonators 821, 841, namely electromechanical coupling coefficients of piezoelectric material used in the parallel resonators 811, 831, 851 and the series resonators 821, 841. For this reason, in the ladder-type piezoelectric filter 8, it may be difficult to assure a desired pass band width. Further, in the ladder-type piezoelectric filter 8, the attenuation of a stop band on a low frequency side or a high frequency side may not be sufficiently assured.
In order to solve these problems, a ladder-type piezoelectric filter 9 shown in a circuit diagram of FIG. 14 has also hitherto been used (see for example, Japanese Patent Application Laid-Open No. 2001-24476). The ladder-type piezoelectric filter 9 is made up by alternately connecting parallel arms 91, 93, 95 and series arms 92, 94. The parallel arms 91, 93, 95 are respectively made up by serially connecting parallel resonators 911, 931, 951 and inductors 912, 932, 952, and the series arms 92, 94 are respectively made up of series resonators 921, 941. The inductors 912, 932, 952 serve to lower resonant frequencies of the parallel resonators 911, 931, 951 so as to widen a pass band width of the ladder-type piezoelectric filter 9. Further, in the ladder-type piezoelectric filter 9, inductances of the inductors 912, 932, 952 are adjusted to make the resonant frequencies of the parallel resonators 911, 931, 951 slightly different from one another, thereby allowing an increase in the attenuation of the stop band on the low frequency side.
However, the ladder-type piezoelectric filter including the parallel arms made up by serially connecting the parallel resonators and the inductors has the problem of not being able to sufficiently assure the attenuation of the stop band in a high frequency side.